This invention relates to an isolation transformer and a transmission control apparatus using the isolation transformer.
The rotary transformer which is one type of the isolation transformer has been frequently used in electric appliances such as video machines.
In an ordinary transformer, two coils are constructed to be rotatable relative to each other, cores having a high relative magnetic permeability are employed to increase the coupling coefficient of the coils and a gap between the cores (coils) is set to an order of several xcexcm. If the coils coupling coefficient is very high, self inductance and mutual inductance of the two coils cancel out each other, and therefore the I/O impedance of a transformer can be designed to be small. Therefore, in the ordinary rotary transformer, impedance matching with a load can be carried out easily.
In such a rotary transformer, if the gap between the cores deflects during a relative rotation between two coils, the coupling condition between the coils is affected. Thus, production accuracy of components must be controlled strictly. Specifically in case of use in an environment having a violent vibration, if the absolute value of the gap is small, the coupling condition of the coil may be largely affected by a minute vibration, which is disadvantageous from the viewpoint of production cost.
On the other hand, if a necessity of transmitting a large-current, large-volume electric energy at a high speed occurs when the isolation transformer is used under a low voltage, impedance matching between the coil and load is very important for the isolation transformer. For this purpose, in the isolation transformer, it can be considered to reduce the equivalent relative magnetic permeability of its magnetic circuit by increasing the gap between the cores, to reduce coil inductance by decreasing the number of windings of the coil, and to reduce DC resistance of the coil, as well other measures. However, because energy is transmitted instantaneously, the transmission frequency needs to be set high. In this case, the higher the frequency, the larger the coil impedance becomes.
The above problems can be solved by suppressing a reduction of the coupling condition between the coils even if the gap between the cores of the isolation transformer is enlarged.
On the other hand, as a non-contact type electric energy transmission apparatus, there is a type using the rotary transformer (a kind of isolation transformer). This kind of the transmission apparatus transmits electric energy supplied from a power source to a load via the aforementioned rotary transformer. For example as disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 6-191373, this apparatus is used as an apparatus for instantaneously activating a shot-firing device (load) of an automotive air bag.
The aforementioned shot-firing device is activated by applying a large current of about several A in a short time of, for example, less than 2-30 m second. As the aforementioned electric energy transmission apparatus, specifically, a rotary transformer, it is required that its transmission efficiency is high enough to achieve a large-current electric energy transmission. Further, the isolation transformer is required to have an excellent high frequency characteristic to achieve an instantaneous electric energy transmission, and generally, it is desirable to set the transmission frequency over about 10 kHz.
From this viewpoint, various considerations have been taken on the isolation transformer and recently, a flat opposing type inductive, isolation transformer has been much expected.
The flat opposing type isolation transformer has a structure in which primary and secondary cores provided with primary and secondary coils respectively, mounted in each of annular concave portions formed in their opposing faces so that they have a symmetrical shape with respect to an axis, are arranged symmetrically in terms of plane via a predetermined gap.
In the isolation transformer having such a structure, a factor important for achieving highly efficient electric energy transmission is coupling efficiency between the aforementioned two coils. For this purpose, it is a requirement to make magnetic flux as large as possible, generated in the primary coil interlink with the secondary coil, and, to reduce leakage of the magnetic flux. Therefore, much effort has been taken to produce the aforementioned cores with a high magnetic permeability material and to reduce the aforementioned gap as much as possible.
However, there is a limitation in reduction of the gap between the cores and there are following problems. That is, even if a fine gap is set, it is very difficult to maintain that gap at a high accuracy because of an influence of vibration, generated heat and the like. For example, if this kind of the isolation transformer is incorporated in a vehicle as a rotary transformer, the opposing distance between the stator and rotor largely changes due to vibration, generated heat and the like. Thus, if the change rate is of the same order as the gap width, the coupling condition of the isolation transformer largely changes so that its electric transmission efficiency largely changes. That is, as the gap is reduced, the change in transmission efficiency due to the gap change is increased. Therefore, it is difficult to raise the transmission efficiency high enough and stabilize the transmission efficiency in the isolation transformer.
Further, in the isolation transformer, if the gap is reduced, the effective permeability of a magnetic path (magnetic circuit) formed by the cores becomes substantially the same order as the magnetic permeability of the core itself. However, because in the isolation transformer, the coil inductance is increased, a high voltage is necessary for realizing a large current transmission. However, because a 12-V battery is exclusively used as a power source of the vehicle, a boosting circuit for a large current as disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 6-191373 is necessary. Therefore, there occurs such a disadvantage that the isolation transformer has a higher cost.
Further, in some type of conventional transmission control apparatuses, the rotary transformer (a kind of isolation transformers) is used in a steering portion of a vehicle to ignite its air bag from the column side in a non-contact manner. For example, Unexamined Japanese Patent Publication (KOKAI) No. 8-322166 has disclosed an idea in which power transmission necessary for air bag ignition and other signal transmission are achieved in interactive ways by using a rotary transformer having a single shaft structure.
In case of ignition of the air bag, the air bag needs to be activated by supplying a current of several A for more than several tens m seconds instantaneously since detection of a collision to a shot-firing device having a resistance as low as 1-3xcexa9 under a low voltage (the vehicle battery is exclusively 12 V).
In case of power transmission necessary for ignition of the air bag, to satisfy this requirement, the aforementioned conventional transmission control apparatus supplies a small power gradually to charge a capacitor provided on the shaft side with a necessary electric power. When an ignition of the air bag is instructed, the aforementioned instruction signal is multiplex-transmitted from the column side to the shaft side via the rotary transformer by carrier wave. If the ignition is necessary after a necessity of the ignition is determined, the aforementioned capacitor is discharged to supply a large current necessary for the ignition thereby activating the shot-firing device. A communication signal from the shaft side, for example, a signal of ON/OFF of a horn (klaxon) switch or the like is multiplex-transmitted via the rotary transformer.
Because in the aforementioned transmission control apparatus, when the ignition of the air bag is instructed, the aforementioned instruction signal is transmitted to the secondary side of the rotary transformer with the carrier wave so as to determine the necessity of an ignition and after that, the aforementioned shot-firing device is activated, there occurs a difference of time between the instruction and a start of supplying a current to the shot-firing device. Particularly in the aforementioned apparatus, because interactive communication is carried out between the shaft side and column side, the transmission direction is controlled by information frame timing adjustment. Therefore, in the aforementioned apparatus, a delay occurs by a frame time at most in the interactive direction and further, a circuit for separating signals to be transmitted in the interactive direction is necessary, thereby leading to complexity of the circuit.
Because in the aforementioned apparatus, a quantity of power for use in power transmission is minute, it takes time to charge the capacitor. Thus, if the capacitor is being charged even when the instance when the air bag is required to be ignited comes, there is a possibility that the ignition is impossible.
The resistance of a shot-firing resistor for use in the shot-firing device is very small as described above. Therefore, to supply a large current instantaneously to the secondary side to feed to the shot-firing resistor, it is necessary to suppress the impedance of the secondary coil and it is desirable to suppress the number of the coil windings.
On the other hand, in communication signal transmission, it is desirable that the impedance of the coil is as high as possible to suppress power consumption. Therefore, the number of the coil windings is desired to be large. Thus, it comes about that there are contradictory favorable impedances.
That is, the aforementioned apparatus selectively uses the frequency by using a relatively high frequency for signal transmission and a relatively low frequency for ignition of the air bag.
The present invention has been achieved in view of the above described problems, and a first object of the invention is to provide an isolation transformer capable of inhibiting a drop of the coupling condition between the coils even if the gap between the cores is enlarged.
A second object of the invention is to provide an isolation transformer having an excellent high frequency characteristic and a high transmission efficiency capable of transmitting a large current of electric energy instantaneously with a simple structure.
A third object of the invention is to provide a transmission control apparatus capable of igniting an air bag surely by supplying a current without a delay of time when the ignition of the air bag is required and further capable of achieving signal transmission between the primary side and secondary side of the isolation transformer effectively.
To achieve the first object, the present invention provides an isolation transformer comprising primary and secondary cores and primary and secondary coils, with the primary coil and the secondary coil being disposed with a gap provided therebetween. Each of the primary coil and the secondary coil is formed of a wire having at least two substantially parallel long sides, and a length of the two long sides in each of the wires is set to be longer than a distance between the two long sides in each of the wires. Each of the wires is wound to have a plurality of turns in a manner such that an outer one of the two long sides of each inner one of the turns is adjacent to an inner one of the two long sides of each respective adjacent outer one of the turns.
Preferably, the primary coil and the secondary coil have an even number of windings in the axial direction or radius direction while a sharp angle formed between a line connecting centers on both ends of an insulating gap between both windings in a cross section of a diameter direction of the coils adjacent in the axial direction or radius direction and a center line of the both coils is in a range of 45xc2x0xc2x125xc2x0.
By using the shielding effect of the coil conductor against magnetic flux, the coupling coefficient between the coils is raised.
At this time, if the primary coil and the secondary coil are combined such that they have an even number of windings in the axial direction or radius direction and, with respect to an insulating gap between both windings in a cross section of a diameter direction of the coils adjacent in the axial direction or radius direction, a line connecting a starting point and an end point of magnetic flux intersecting each coil is in a range of 45xc2x0xc2x125xc2x0 relative to the center line of both the coils, a horizontal factor in the diameter direction of magnetic flux intersecting each coil and a vertical factor in the coil center line direction intersecting the former come to intersect the conductor surface of each coil substantially perpendicularly. As a result, the conductor surface area perpendicular to the conductor increases so that the eddy current also increases, thereby producing a large shielding effect.
The surface effect of the conductor has been well known. The surface effect of the conductor refers to a phenomenon that a current in the conductor is concentrated on the surface corresponding to the frequency. The higher the frequency, the more current is concentrated. Further, the shallower from the surface, the larger density of current flowing in that portion is. For example, in case of alternating signal of 10 KHz, current is concentrated within about 0.5 mm from the conductor surface. Thus, if the depth is sufficient, the shielding effect of the conductor is intensified more as the conductor surface area perpendicular to the magnetic flux is increased.
On the other hand, to achieve the second object, in the isolation transformer of the present invention, the effective magnetic permeability of a magnetic circuit formed by the cores is reduced appropriately so as to stabilize the transmission efficiency. Further, in the isolation transformer of the present invention, by increasing magnetic resistance against leakage magnetic flux, the leakage magnetic flux is suppressed so as to intensity the electric energy transmission efficiency.
Particularly in the isolation transformer of the present invention, the position of a gap formed between the primary core and the secondary core is different from a position of a gap formed between the primary coil and the secondary coil. The aforementioned second object is achieved, for example, by disposing the primary coil and secondary coil at a position where they are wrapped by one of the primary core and secondary core, without a reduction of the gap.
Further, the other isolation transformer of the present invention comprises a ring-like shielding body made of a high conductivity material having a slit for interrupting a closed loop. For example, by providing the aforementioned ring-like shielding body in a direction intersecting the leakage magnetic flux between the coils, the leakage magnetic flux is reduced so as to achieve the second object.
In the other isolation transformer of the present invention, the position of a gap formed between the cores is different from the position of a gap formed between the coils and a ring-like shielding body is disposed to intersect a traveling direction of magnetic flux interlinking between the coils. As a result, a large current electric energy can be transmitted in a high efficiency.
Further, the present invention provides an isolation transformer comprising a primary core, a secondary core disposed to oppose the primary core via a predetermined gap, and primary coil and secondary coil attached to the primary core and secondary core respectively such that they are inductively coupled, wherein, of the primary core and the secondary core, one thereof is a disc like member having an outer peripheral wall on a peripheral edge while the other is a disc like member having a cylindrical portion to be disposed inside the outer peripheral wall in the center, and of the primary coil and the secondary coil, one thereof is disposed along an inside face of the outer peripheral wall of the one core while the other is disposed along an outside face of the cylindrical portion of the other core, and the position of a gap formed between the primary core and the secondary core is different from the position of a gap formed between the primary coil and the secondary coil.
To achieve the aforementioned third object, the present invention provides a transmission control apparatus including an isolation transformer comprising plural primary coils and plural secondary coils separately attached to the primary core and secondary core respectively such that they are inductively coupled, a high output signal transmission means connected to one primary coil of the primary coils and one secondary coil inductively coupled to that primary coil for transmitting the high output signal for igniting an air bag, and a low output signal transmission means connected to the other primary coil of the primary coils and the secondary coil inductively connected to that primary coil for transmitting low output signal for information transmission. For example, in case where the low output signal includes plural kinds of signals, the signal transmission circuit transmits each low output signal with a different resonant frequency to the isolation transformer.
That is, the power transmission system for transmitting from the column side to the air bag shot-firing circuit on the shaft side and the signal transmission system for transmitting from the shaft side to the column side are separated. As a result, the high output signal and low output signal can be transmitted at the same time via the isolation transformer connected to each transmission system, so that plural low output signals are transmitted, thereby achieving instantaneous air bag ignition and improving signal transmission efficiency.
On the other hand, preferably the transmission control apparatus comprises a plurality of the low output signal transmission means, the other primary coil and the other secondary coil each comprising plural coils corresponding to the number of the low output signal transmission means and being attached to the primary core and the secondary core separately such that they are inductively coupled with each other, the low output signal transmission means being connected to the corresponding primary coil and the secondary coil inductively coupled with the primary coil so that the low output signal is transmitted via the primary coil and secondary coil.
Preferably, the primary core and secondary core are formed of material having a different relative magnetic permeability depending on a use purpose of a signal to be transmitted through the plural primary coils and secondary coils.
Preferably, core of material having a high magnetic permeability is disposed in a path of interlinkage magnetic flux between the coils and a sectional area perpendicular to the interlinkage magnetic flux of the core is different depending on power level of the signal.
Here, in case of transmitting electric signal or electric power using the transformer, usually, the primary side and secondary side are distinguished depending on the transmission direction. That is, electric signal or electric power is transmitted from the primary side to the secondary side. However, in the isolation transformer of the present invention, interactive transmission can be considered as an object. Thus, for convenience of description in this specification, it is defined that a side of supplying a power is the primary side and a side of receiving the power is the secondary side based on the power transmission direction of the isolation transformer.